The present invention relates to a heat resistant steel suitable for use as a material of valves on internal combustion engines such as automotive engines.
Hitherto, 21-4N steels (0.55C-0.2Si-9Mn-4Ni-21Cr-0.4N) (by weight percent), known as high manganese heat resistant steels, have been used as materials of exhaust valves of internal combustion engines, because of various advantages such as excellency in high temperature strength, resistance to corrosion by lead and sulfur contained in gasoline and oxidation resistance, as well as low cost. In recent years, however, there is an increasing demand for heat resistant steels for engine valves having greater high-temperature strength than 21-4N steels (containing 21% chromium and 0.4% nitrogen), in view of a current trend for higher efficiency and higher power of gasoline engines which inevitably leads to higher combustion temperature. In order to cope with this demand, various steels have been proposed in, for example, Japanese Unexamined Patent Publication Nos. 55-2775, 60-77964, 59-211557, 63-89645 and 1-79351 (1989).
Those proposed steels, which are intended to improve high-temperature strength of 21-4 N steel, have a greater carbon content, as well as greater content of alloying elements such as vanadium, niobium, molybdenum and tungsten, than 21-4N steel. In order to attain a greater high-temperature strength, it is preferred that these alloying elements are dissolved in the matrix or, alternatively, precipitated in the form of fine carbides to strengthen the structure. In other words, it is not preferred that these alloying elements exist in the form of coarse carbides. In those improved steels, therefore, it is necessary to conduct a solid solution treatment at 1100.degree. C. to 1150.degree. C. which is higher than 1050.degree. C. at which a solution treatment for 21-4N steel is usually conducted, in order to attain the desired high-temperature strength.
Solution treatment in such higher temperature inevitably leads to an increase in the heat consumption causing a damage of the structure of the treating furnace, requiring an improvement in existing treating systems and, hence, a rise in the cost of production of engine valves.
Accordingly, a development of a steel, which can provide a sufficiently large high-temperature strength even by a solution treatment conducted at conventional treating temperature of 1050.degree. C. or so, has been longed for.
It is also to be pointed out that the improved steels mentioned above are rich in elements which adversely affect a property of oxidation resistance, e.g., vanadium and niobium, in order to develop greater high-temperature strength. Consequently, those improved steels exhibit oxidation resistance inferior to that exhibited by 21-4N steel.
More specifically, disadvantages of the known steels proposed for improving 21-4N steel are as follows.
The steel proposed in Japanese Unexamined Patent Publication No. 61-20623 exhibits inferior oxidation resistance to that of 21-4N steel, due to too excessive amounts of vanadium and niobium. The steel proposed in Japanese Unexamined Patent Publication No. 60-77964 also exhibits inferior oxidation resistance to that of 21-4N steel, due to excessive amounts contents of vanadium and niobium. In addition, this steel has a large carbon content so that the strength is impaired due to insufficient solution of coarse primary carbides when it is solution-treated at the same temperature as that for 21-4N steel, though it exhibits an appreciably large high-temperature strength when solution-treatment at a high temperature is conducted.
The steel disclosed in Japanese Unexamined Patent Publication No. 59-211557 also exhibits an unsatisfactorily low level of oxidation resistance due to containment of vanadium as an essential component. Both carbon and nitrogen are contained in this steel for the purpose of forming carbon-nitrides. The disclosure, however, fails to show any definite content of nitrogen. In addition, the carbon content is as high as 0.65% to 0.72% so that the high-temperature strength cannot be improved due to insufficient solid-solution of primary carbides when the solution treatment is conducted at the conventional low temperature.
Steels disclosed in Japanese Unexamined Patent Publication Nos. 63-89645 and 1-79351 (1989) also suffer a disadvantage in that the desired high-temperature strength cannot be obtained when those steels are treated at the conventional low temperatures, due to the large carbon content. Furthermore, molybdenum and tungsten used as the solution-strengthening elements cannot provide satisfactory effects.